1. Field of the Invention
This invention relates to a display device of the kind in which electrical signals are selectively applied to a material to cause local changes in an optical characteristic of the material.
The invention is particularly concerned with such a display device in which the electrical signals are applied in matrix fashion to a suitable material. A material in which visible images may be formed by chemically-produced colour changes as a result of the selective use of electrical potentials, is hereinafter called an electrochromic material. With many electrochromic materials, the images are reversible, that is, they can be erased by applying a reverse potential to the imaging material.
2. Description of the Prior Art
In one kind of matrix electrode configuration the electrodes used to selectively apply potentials across the material are in the form of two spaced sets of parallel conductive strips which overlie one another in an orthogonal matrix configuration. By applying potentials to the appropriate strip of each set, it is possible to address any point in the material defined by an intersection of two strips. By making each applied potential less than the coloration threshold potential for the material but greater than one half of the threshold potential, it is possible to cause coloration only at the selected intersection.
This configuration suffers from the disadvantage that in order to pass sufficient charge through the electrochromic material to cause coloration in a usefully short time it is necessary to pass a large current pulse. Although this can be achieved, the necessary circuitry is expensive and difficulty may be experienced in fabricating electrodes of sufficiently high conductivity to cope with such currents.
In our copending U.S. Pat. No. 4,175,836, issued Nov. 29, 1979, there is described and claimed a method and apparatus for causing coloration of an electrochromic material which includes passing through the material an electrical pulse of the polarity which tends to cause coloration and at a potential above the threshold potential for coloration of the material, followed by a DC potential of the same polarity but of magnitude less than the threshold potential. The pulse may contain considerably less than the total electrical charge required to cause coloration, and this gives rise to the advantage that it is possible to manufacture the necessary electronics much more cheaply than if the full charge required to cause coloration were applied to the matrix.
Although the invention of the above mention patent specification makes possible a reduction in the cost of a matrix-addressed display device, it is still necessary to have a large number of separate matrix-addressing lines. Thus in, for example, a 100.times.100 matrix, 200 lines are needed.
In Strom U.S. Pat. No. 3906451 there is disclosed an apparatus and method for erasing selected gas discharge cells by the use of non-coincident pulses.